1. Field of the Invention
The present invention relates to a gas sensor for measurement of a particular gas component in a to-be-measured gas and, in particular, to a nitrogen oxide sensor for measurement of NOx as a to-be-measured gas component.
2. Description of the Related Art
Various measurement methods and measurement apparatuses have been developed in order to know the concentration of a particular gas component in a to-be-measured gas. For example, there is known a gas sensor of limit current type which uses an ion-conductive solid electrolyte. Further, as a method for measurement of NOx in a to-be-measured gas (e.g. a combustion gas), there is known a technique which uses a sensor comprising an oxygen ion-conductive solid electrolyte (e.g. zirconia) and a Pt electrode and a Rh electrode both formed on the solid electrolyte and measures an electromotive force between the two electrodes utilizing the reducing ability of Rh for NOx (for example, Patent Literature 1).
In such a sensor, the electromotive force changes greatly for the change in the concentration of oxygen in a combustion gas (a to-be-measured gas) but the change of the electromotive force is small for the change in the concentration of to-be-measured gas component (e.g. NOx); therefore, the measurement result of concentration tends to be influenced by various factors.
Patent Literature 1: JP-B-2000-28576
When such a gas sensor is fitted to the exhaust system of internal combustion engine (e.g. automotive engine) and the internal combustion engine is operated, there has been a problem that the sensitivity of gas sensor to to-be-measured gas component (e.g. NOx) reduces gradually. The present inventors investigated on the reasons therefor and, as a result, it was found that the harmful substances (e.g. Mg) contained in the exhaust gas of internal combustion engine have a large influence.
The aim of the present invention is to provide a gas sensor and a nitrogen oxide sensor, which, when fitted to, for example, the exhaust system of internal combustion engine, can suppress the influence of harmful substances (e.g. Mg) contained in a to-be-measured gas and can prevent the reduction in sensitivity with the increase in use time.